The present invention relates to a method of preparing (trifluoromethyl)pyridine compounds in a highly selective manner by reacting (trichloromethyl)pyridine compounds with mercury or silver fluorinating agents in a liquid phase halogen exchange reaction.
Fluorination of (trichloromethyl)pyridine compounds has been carried out by vapor phase fluorination which requires the use of high temperatures. Such vapor phase reactions suffer from disadvantages including, for example, energy costs associated with elevating the temperature of the reactants. Additionally, at the temperature necessary for the fluorination reaction, both the starting materials and the end products can be decomposed or converted into undesirable by-products which lead to low conversions and low selectivities to the desired (trifluoromethyl)pyridine products. See, for example, U.S. Pat. Nos. 4,266,064 and 4,288,599.
Another method of preparing (trifluoromethyl)pyridine compounds has been to contact a (trichloromethyl)pyridine compound with antimony trifluorodichloride (SbF.sub.3 Cl.sub.2). See, for example, U.S. Pat. Nos. 3,136,822; 3,787,420; and 3,818,019. The disadvantages associated with this reaction are, for example, the difficulty in controlling an extremely exothermic reaction, the high cost of antimony trifluoride and the difficulty in recycling the antimony by-product. Thus, it is evident that a more efficient method of preparing (trifluoromethyl)pyridine compounds is desirable.
It has been unexpectedly found that mercury and silver fluorinating agents convert (trichloromethyl)pyridine compounds into (trifluoromethyl)pyridine compounds in high yields with a high selectivity to the desired (trifluoromethyl)pyridine compounds in liquid phase halogen exchange reactions conducted at low temperatures.